Saturday, April 2, 2011

Pediatric Thyroid Cancer: Epidemiology

United States: Thyroid cancer, the most common pediatric endocrine neoplasm, represents 1-1.5% of all pediatric malignancies and 5-5.7% of malignancies in the head and neck. Only 5% of all thyroid cancers occur in children and adolescents. Thyroid nodules occur in 4-7% of the general adult population and in only 1-2% of the pediatric population. These numbers are estimated using a compilation of data from multiple reports.

Paradoxically, despite the lower incidence of thyroid nodules in children, a pediatric thyroid nodule has a greater risk of containing or developing a malignancy. Whereas 5% of nodules in adults are malignant, in the pediatric population, the percentage of malignant nodules is 26.4%. The incidence of malignancy in multinodular goiter is 1-7% and 10-25% in solitary nodules. Pediatric thyroid cancer (3% prevalence) in adolescents is also associated with juvenile autoimmune thyroiditis.

Papillary thyroid cancer is by far the common thyroid malignancy in children. Although papillary carcinoma is more aggressive in children than in adults, pediatric papillary cancer carries a much better prognosis that adult thyroid cancer.

Medullary thyroid cancer (MTC), which constitutes 5% of pediatric thyroid malignancies, is usually associated with multiple endocrine neoplasia type 2 (MEN2) in the pediatric population. The inheritance pattern occurs either sporadically or as familial MTC without other associated endocrine abnormalities. MEN2 consists of MTC and pheochromocytoma and either hyperparathyroidism (2A) or mucosal neuromas (2B). MTC associated with MEN2B is more virulent and may occur and metastasize early in infancy.

International: After the Chernobyl nuclear power plant disaster, individuals living in Russia, Ukraine, and Belarus were exposed to significant levels of radioactive iodines, primarily 131I. This radioactivity, which is concentrated in the thyroid gland, has resulted in a substantial increase in pediatric thyroid cancer rates among this cohort of children.

Mortality/Morbidity: Pediatric thyroid malignancies are usually a well-differentiated papillary subtype or the papillary-follicular subtype, but all histologic types have been observed. Children commonly present with advanced disease. At presentation, 70% of patients have extensive regional nodal involvement, and 10-20% of patients have distant metastasis. The lungs are the most common sites of metastasis.

Pediatric patients seem to have higher local and distant recurrence rates than adults, but they tend to respond rapidly to therapy. The prognosis for children is excellent, with mortality rates of less than 10%. Benign tumors such as follicular adenomas should be considered at risk for tumor progression toward follicular thyroid carcinoma, and they must be surgically addressed.

Sex / Gender: Thyroid carcinoma is 2-3 times more common in females. The gender distribution of thyroid carcinoma differs between adults and children. Thyroid cancer is 4 times as common in women as in men. This difference is not seen in individuals younger than 15 years; the girl-boy ratio is as low as 1.5:1. However, in individuals aged 15–20 years, the female-to-male ratio is 3:1. This implies that female sex hormones, especially during puberty, play a significant yet still undefined role in the increased incidence of thyroid cancer in females.

Age: Age is a major determinant of both the incidence and recurrence of pediatric thyroid carcinoma. Pediatric thyroid carcinoma occurs more frequently in adolescents, although it has been reported in the neonatal period. In children younger than 10 years, identified thyroid lesions are more likely to be malignant. Children younger than 10 years are also more likely to have recurrent cancer.